Light emitting device and methods for forming the same

ABSTRACT

The present invention provides a light emitting device, which comprises an epitaxial stack structure, a II/V group compound contact layer directly formed on the epitaxial stack structure, a protrusion or recess type structure directly formed on the II/V group compound contact layer, and a conductive layer covering the protrusion or recess type structure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of pending U.S. patent applicationSer. No. 11/848,458, filed on Aug. 31, 2007, entitled “LIGHT EMITTINGDEVICE AND METHODS FOR FORMING THE SAME,” which is incorporated hereinby reference and assigned to the assignee herein.

TECHNICAL FIELD

The present invention relates to a light emitting device, and moreparticularly to a light emitting device taking a II/V group compoundcontact layer as the ohmic contact layer, and having an epitaxiallygrown nitride-crystallized layer of a specific structure on the II/Vgroup compound contact layer. Thereby the light from the active layermay be more efficiently outputted, and thus the light emittingefficiency of the light emitting device is improved.

BACKGROUND OF THE INVENTION

In FIG. 1, the epitaxially grown epitaxial layer on the substrate 100includes a first compound semiconductor layer 110, an active layer 120and a second compound semiconductor layer 130. The recombination ofminority carriers may bring energy conversion when the current flowsforward through the active layer 120 between the second compoundsemiconductor layer 130 and the first compound semiconductor layer 110,thereby the epitaxial layer is capable of emitting light. An ohmiccontact layer 140 is on the epitaxial layer and serves to enhance thecurrent diffusion and the contact between a first electrode 150 and thesecond compound semiconductor layer 130. A second electrode 160 on thefirst compound semiconductor layer 110 forms a current loop with thefirst electrode 150, the ohmic contact layer 140, the second compoundsemiconductor layer 130, the active layer 120, and the first compoundsemiconductor layer 110.

The light emitted from the epitaxial layer due to the energy conversionmay go through the ohmic contact layer 140, and the light intensity maybe reduced, since the ohmic contact layer 140 may block a portion of thelight. Therefore, a light emitting device with improved light intensityis needed.

SUMMARY OF THE INVENTION

The present invention provides a light emitting device, which comprisesan epitaxial stack structure, a II/V group compound contact layerdirectly formed on the epitaxial stack structure, a protrusion or recesstype structure directly formed on the II/V group compound contact layer,and a conductive layer covering the protrusion or recess type structure.

The object, embodiments, features and advantages would be more apparentas referring to the description of the preferred embodiments and thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a light emitting diode according tothe prior art;

FIG. 2A shows a picture of a crystallized layer of a protrusion typestructure according to the present invention taken by a scanningelectron microscope;

FIG. 2B shows a picture of a crystallized layer of a recess typestructure according to the present invention taken by a scanningelectron microscope;

FIG. 3A is a cross-sectional view of a light emitting device having aII/V group compound contact layer and a nitride nucleus layer of aprotrusion type according to the present invention;

FIG. 3B is a cross-sectional view of a light emitting device having aII/V group compound contact layer and a nitride nucleus layer of arecess type according to the present invention;

FIG. 4A is a cross-sectional view of a light emitting device having anitride nucleus layer and a nitride-crystallized layer of a protrusiontype according to the present invention; and

FIG. 4B is a cross-sectional view of a light emitting device having anitride nucleus layer and a nitride-crystallized layer of a recess typeaccording to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Some embodiments of the present invention would be described in detailas below. However, the present invention may be implemented in otherembodiments in addition to which are described herein. And the scope ofthe present invention is not limited to the description, but should beconstrued according to the appended claims.

First referring to FIGS. 2A and 2B, the pictures of nitride-crystallizedlayers of a protrusion type structure and a recess type structure takenby a scanning electron microscope are respectively shown.

Then referring to FIG. 3A, a schematic diagram of a light emittingdevice according to the present invention is shown. The light emittingdevice includes a substrate 10, an epitaxial stack structure 20 having afirst portion 20A and a second portion 20B over the substrate 10, a II/Vgroup compound contact layer 30 on the first portion 20A of theepitaxial stack structure 20, and a nitride-crystallized layer 42A onthe II/V group compound contact layer 30. The nitride-crystallized layer42A may be of a protrusion type structure (as shown in FIG. 2A) or arecess type structure (as shown in FIG. 2B).

Still referring to FIG. 3A, the light emitting device disclosed in thepresent invention further includes a transparent conductive layer 50covering the nitride-crystallized layer 42A, a first electrode 60 of afirst conductive type on a portion of the transparent conductive layer50, and a second electrode 70 of a second conductive type on the secondportion 20B of the epitaxial stack structure 20. The substrate 10 may bea transparent substrate or a sapphire substrate generally employed inthe light emitting devices. The epitaxial stack structure 20 furtherincludes a first semiconductor layer 22 of a first conductive type onthe substrate 10, an active layer 24 on the first semiconductor layer 22of the first conductive type, and a second semiconductor layer 26 of asecond conductive type on the active layer 24. Here the firstsemiconductor layer 22 may be made of GaN or a semiconductor layerincluding GaN-related compounds. The active layer 24 may include quantumwell or multi-quantum well and may be made of GaN or a semiconductorlayer including GaN-related compounds. The first semiconductor layer 22and the second semiconductor layer 26 are of opposite conductive types,and may be n-type and p-type GaN semiconductor layers, respectively.Besides, the first conductive type of the first electrode 60 is oppositeto the second conductive type of the second electrode 70.

Furthermore, the first electrode 60 is structurally separated from thestructure over the first portion 20A of the epitaxial stack structure20. In this embodiment, a II/V group compound contact layer 30 isemployed to eliminate the difficulty of forming p-type ohmic contactlayer. Besides, the formation of the ohmic contact and the externalquantum efficiency of the light emitting devices could be improved bythe nitride-crystallized layer of 3-dimensional specific structure 42Aor 42B epitaxially grown on the II/V group compound contact layer 30.

Again referring to FIGS. 3A and 3B, the present invention furtherprovides a method of forming the light emitting device including thefollowing steps. A substrate 10 is provided. An epitaxial stackstructure 20 is epitaxially grown on the substrate 10, wherein theepitaxial stack structure 20 has a first portion 20A and a secondportion 20B. Here the method for forming the epitaxial stack structure20 may include epitaxially growing a first semiconductor layer 22 of afirst conductive type on the substrate 10, forming an active layer 24 onthe first semiconductor layer 22, and forming a second semiconductorlayer 26 of a second conductive type on the active layer 24.

Next, a II/V group compound contact layer 30 is formed on the firstportion 20A of the epitaxial stack structure 20. Here the II groupelements of the II/V group compound contact layer 30 may include Zn, Be,Mg, Ca, Sr, Ba, Ra. The V group elements of the II/V group compoundcontact layer 30 may include N, P, As, Sb Bi. The II group and V groupelements mentioned above may be arbitrarily compounded to form the II/Vgroup compound contact layer 30 implemented in the present invention. Inthis embodiment, the II/V group compound contact layer 30 may be made ofMg_(x)N_(y). The Mg_(x)N_(y) may be formed by metal organic chemicalvapor deposition (MOCVD) with a precursor containing magnesium, such asDCp₂Mg(bis(cyclopentadienyl)Magnesium) orBis(methylcyclopentadienyl)Magnesium, reacting with introduced NH₃ inthe reaction chamber. Here the II/V group compound contact layer 30 maybe a Mg_(x)N_(y) layer formed on the epitaxial stack structure 20 with athickness of about 75 Å and a roughness smaller than about 10 nm, andpreferably, about 2 nm. The II/V group compounds can be grown on theepitaxial stack structure 20, especially on p-type compounds, and have aband-gap energy smaller than the conventional III/V group compounds.Several examples of the band-gap energy of the II/V group compounds maybe as follows: 0.93 eV for Zn₃As₂, 3.2 eV for Zn_(n)N, 1.57 eV for Zn₃P₃and 2.8 eV for Mg₃N₂. However, the GaN as conventional III/V groupcompound contact layer has a band-gap energy of about 3.34 eV.Therefore, due to the smaller band-gap energy, the II/V group compoundcontact layer 30 would provide a good ohmic contact and thus a suitableohmic electrode.

Afterward, the nitride-crystallized layer 42A and 42B of 3-dimensionalspecific structure are epitaxially grown on the II/V group compoundcontact layer 30. The brightness of light from the active layer (oractive area) 24 and thus the light emitting efficiency could be improvedthrough the 3-dimensional specific structure. The nitride-crystallizedlayer 42A of a protrusion type structure (as shown in FIG. 3A) or thenitride-crystallized layer 42B of a recess type structure (as shown inFIG. 3B) on the II/V group compound contact layer 30 may be obtained bycontrolling the conditions of epitaxial growth, such as growthtemperature, growth pressure or molar ratio of V group to III groupelements. For example, the nitride-crystallized layer 42A of theprotrusion type structure (as shown in FIG. 2A) may be formed undertemperature of about 800 to 1000° C., pressure of about 76 to 700 torr,and a molar ratio of V group to III group elements as about 500:1000.The nitride-crystallized layer 42B of the recess type structure (asshown in FIG. 2B) may be formed under temperature of about 600 to 900°C., pressure of about 76 to 700 torr, and a molar ratio of V group toIII group elements as about 500:1000.

Next, a transparent conductive layer 50 is formed on thenitride-crystallized layer of the protrusion type structure 42A or therecess type structure 42B. The transparent conductive layer 50 may bemade of ITO, ZnO, Ni/Au alloy, CTO, TiWN, In₂O₃, SnO₂, CdO, ZnO, CuGaO₂or SrCu₂O₂. Finally, a first electrode 60 is formed on a portion of thetransparent conductive layer 50 to form the ohmic contact, and a secondelectrode 70 is formed on the second portion 20B of the epitaxial stackstructure 20. Please note that the second electrode 70 may be formedafter the epitaxial layers 22, 24, 26 are etched to expose a portion ofthe first semiconductor layer 22.

Referring to FIGS. 4A and 4B, another preferred embodiment of the lightemitting device disclosed by the present invention is shown. Thesubstrate 10, the epitaxial stack structure 20, the II/V group compoundcontact layer 30, the transparent conductive layer 50, the firstelectrode 60 and the second electrode 70 disclosed in FIGS. 4A and 4Bhave similar structures and functions to those shown in FIGS. 3A and 3B,and would not be elaborated herein.

As shown in FIG. 4A, the light emitting device includes a substrate 10,an epitaxial stack structure 20 having a first portion 20A and a secondportion 20B on the substrate 10, a II/V group compound contact layer 30on the first portion 20A of the epitaxial stack structure 20, a nitridenucleus layer 41 on the II/V group compound contact layer 30, anitride-crystallized layer 42A of a protrusion type structure on thenitride nucleus layer 41 and a transparent conductive layer 50 coveringthe nitride-crystallized layer 42A and the nitride nucleus layer 41. Thelight emitting device shown in FIG. 4B is similar, while thenitride-crystallized layer 42B is of a recess type structure.

Besides, the light emitting device may further include a first electrode60 of a first conductive type on the transparent conductive layer 50,and a second electrode 70 of a second conductive type on the secondportion 20B of the epitaxial stack structure 20. The first and secondconductive types are opposite to each other. The second electrode 70 isstructurally separated from the structure on the first portion 20A ofthe epitaxial stack structure 20. It should be noted that, the epitaxialstack structure 20 bottom-up includes: a first semiconductor layer 22 ofa first conductive type on the substrate 10, an active layer 24 on thefirst semiconductor layer 22, and a second semiconductor layer 26 of asecond conductive type on the active layer 24. Here the firstsemiconductor layer 22 and the second semiconductor layer 26 are ofopposite conductive types.

Furthermore, the present invention provides a method of forming lightemitting device including the following steps. A substrate 10 isprovided. An epitaxial stack structure 20 is epitaxially grown on thesubstrate 10, wherein the epitaxial stack structure 20 has a firstportion 20A and a second portion 20B. Here the method for forming theepitaxial stack structure 20 may include forming a first semiconductorlayer 22 of a first conductive type on the substrate 10, forming anactive layer 24 on the first semiconductor layer 22, and forming asecond semiconductor layer 26 of a second conductive type on the activelayer 24. The epitaxial layers 22, 24, 26 may be etched to expose aportion of the first semiconductor layer 22, thus the first portion 20Aand the second portion 20B are formed. Next, a II/V group compoundcontact layer 30 is formed on the first portion 20A of the epitaxialstack structure 20.

Afterward, a nitride nucleus layer 41 easy to form on the II/V groupcompound contact layer 30 is epitaxially grown on the II/V groupcompound contact layer 30. Then taking the nitride nucleus layer 41 as aseed layer, a nitride-crystallized layer 42A of a protrusion typestructure (as shown in FIG. 4A) or a nitride-crystallized layer 42B of arecess type structure (as shown in FIG. 4B) is epitaxially grown on theII/V group compound contact layer 30. The brightness of light from theactive layer (or active area) 24 and thus the light emitting efficiencycould be improved through no matter the nitride-crystallized layer ofthe protrusion type structure 42A or the nitride-crystallized layer ofthe recess type structure 42B. It should be noted that, the nitridenucleus layer 41 may be made of the same materials as thenitride-crystallized layer 42A or 42B, which could be a semiconductorincluding III group compounds, such as quaternary compound AlInGaN orternary compound p-InGaN. Further, a transparent conductive layer 50 isformed to cover the nitride-crystallized layer 42A or 42B and thenitride nucleus layer 41.

Moreover, the method of forming light emitting device may furtherinclude: forming a first electrode 60 of a first conductive type on aportion of the transparent conductive layer 50 to form ohmic contact,and forming a second electrode 70 of a second conductive type on thesecond portion 20B of the epitaxial stack structure 20. The secondelectrode 70 is structurally separated from the structure on the firstportion 20A of the epitaxial stack structure 20. Besides, the firstelectrode 60 and the second electrode 70 are of opposite conductivetypes.

According to the above descriptions, the II/V group compound contactlayer 30 of the present invention takes the place of the conventionalp-type semiconductor layer being the ohmic contact layer. Additionally,since a III group nitride nucleus layer 41 is easy to form on the II/Vgroup compound contact layer 30, a V/III/II group compound crystallizedlayer could be formed on the II/V group compound contact layer 30.

Furthermore, taking the nitride nucleus layer 41 as the seed layer, thenitride-crystallized layer 42A or 42B of the specific structure isepitaxially grown. The brightness of light from the active layer 24 andthus the light emitting efficiency of the light emitting device could beimproved through the protrusion or recess type structures of thenitride-crystallized layer 42A or 42B.

The above description is only for preferred embodiments, but not tolimit the scope of the present invention. Any other equivalent changesor modifications performed with the spirit disclosed by the presentinvention should be included in the appended claims.

We claim:
 1. A light-emitting device, comprising: an epitaxial stackstructure; a II/V group compound contact layer directly formed on theepitaxial stack structure; a protrusion or recess type structuredirectly formed on the II/V group compound contact layer; and aconductive layer covering the protrusion or recess type structure. 2.The light-emitting device of claim 1, wherein the II/V group compoundcontact layer comprises Mg_(x)N_(y).
 3. The light-emitting device ofclaim 1, wherein the conductive layer comprises a material selected fromthe group consisted of ITO, ZnO Ni/Au alloy, CTO, TiWN, In₂O₃, SnO₂,CdO, ZnO, CuGaO₂, and SrCu₂O₂.
 4. The light-emitting device of claim 1,further comprising: a nucleus layer formed between the II/V groupcompound contact layer and the protrusion or recess type structure. 5.The light emitting device of claim 4, wherein the nucleus layercomprises nitride.
 6. The light-emitting device of claim 4, wherein thenucleus layer and the protrusion or recess type structure comprise anidentical material.
 7. The light-emitting device of claim 1, wherein theconductive layer is transparent to light from the epitaxial stackstructure.
 8. The light-emitting device of claim 1, further comprising atransparent substrate connected to the epitaxial stack structure.